1. Field of the Invention
This invention relates generally to motor control systems, and more particularly to a variable speed drive for a polyphase induction motor in which speed control is provided by a thyristor frequency converter which drives the motor at lower speeds in a single phase mode.
2. Description of the Prior Art
Polyphase induction motors of the squirrel cage and wound rotor types are widely used in a variety of industrial applications. While the polyphase induction motor is well adapted for constant speed work, there are many applications where variable speed control is necessary. Of particular interest are variable speed applications involving fan-type loads, i.e., the type of load encountered in transporting a liquid or gas where the load torque varies as the square of the motor speed. Examples of such fan type loads are blower drives for forcing furnace drafts and pump drives for oil and gas pipelines.
Some fan type loads require single speed operation, but others, such as those associated with boiler draft control, may require a speed range of 2:1 or more in order to control the transported volume of air. Conventional electrical methods of speed control have sometimes been used for these applications, but they are relatively expensive and are thus in competition with mechanical speed control devices such as hydraulic couplings or other means of volume control such as damper vanes. Both mechanical speed control arrangements and damper vanes are relatively inexpensive but are inherently inefficient at reduced volumes. Since the drive for fan type loads may be required to operate for extended periods at reduced speeds, the question of efficiency is of interest. It is desirable, therefore, to provide an electrical speed control system for a polyphase motor which is cost competitive with mechanical speed control arrangements and which operates efficiently at reduced speed levels.
A large number of electrical drives have been developed over the years for the purpose of securing a wide range of operating speed from an induction motor. These drives generally involve one of the following methods: changing the frequency of the applied stator voltage; changing the number of poles of both the stator and the rotor; controlling the rotor speed by means of rheostatic rotor control; or mounting the stator in bearings and driving it with an auxiliary motor. These control methods depend upon switching in the power circuits or modulation of the power flow by intermediate power conversion equipment. The cost and complexity of the required auxiliary equipment have limited the application of these electrical drive systems in favor of the simple and relatively inexpensive mechanical control devices. Recently, however, improvements in thyristor switching devices such as the silicon control rectifier (SCR) have made possible static frequency converters which are inexpensive, reliable, and efficient. This has revived interest in the possibility of using electric motor drives for speed control of induction motors in applications which traditionally have been controlled by mechanical means.